Technical Abstract:
When alfalfa is killed, some of the accumulated N in the soil and in alfalfa leaves, stems, and roots becomes available to subsequent crops. This increased N supply is known as the “alfalfa N credit,” which is the amount of fertilizer or available manure N a grower can save, resulting in higher net return per acre. To better understand how growers are managing fertilizer and manure N in corn after alfalfa and the reasons why growers may or may not accept alfalfa N credits, researchers asked Minnesota farm operators about their nutrient management. A third of the recipients responded and, after excluding ineligible surveys, 528 complete responses were used in the analysis. About one-third (36%) reported that they planted first-year corn following a good alfalfa stand, 45% followed fair alfalfa stands, and 19% followed poor alfalfa stands. Based on University of Minnesota Extension guidelines, growers with no manure application during alfalfa termination reduced N fertilizer rates sufficiently to first-year corn 50% of the time. Of those who applied manure when terminating alfalfa, one-third (31%) reported reducing total N rates (fertilizer and manure) to first-year corn according to these guidelines. About 72% of respondents grew a second consecutive crop of corn after alfalfa. Nearly 40% of the growers without manure application to either first- or second-year corn reduced N fertilizer rates to second-year corn, but only 19% of those who applied manure for either the first- or second-year corn or for both crops reduced total N rates (manure and fertilizer). Limited confidence in the guidelines themselves may be a constraint. Growers appear to rely more heavily on sources other than University of Minnesota Extension for this information, including past experience, soil testing labs, fertilizer dealers, and independent crop consultants. Improved prediction and adoption of alfalfa N credits are needed to help growers improve profit and reduce the risk of N losses.